1. Field of the Invention
The present invention relates to a traction control system for eliminating acceleration slip of wheels or so-called wheel spinning upon acceleration of the vehicle, and in particular to a traction control system which stops the supply of fuel to an on-board engine so as to eliminate an acceleration slip of the wheels by the fuel cut.
2. Description of Related Art
Acceleration slip of the wheels is generally caused when the drive force of the wheels becomes excessive in view of the relationship to the frictional coefficient of the road surface. Thus, it is possible to eliminate acceleration slip by lowering the driving force to the wheels.
A typical measure for lowering the drive force to the wheels is to lower the output power of the engine that drives the wheels. Thus, there has been proposed a traction control system wherein the output power of the engine is lowered by fuel-cut for stopping the supply of fuel to the engine so as to eliminate the wheel spinning. Such a traction control system is disclosed in JP 10-35329A, for example.
Needless to say, when the engine speed during the fuel-cut is lowered down to a fuel recovery speed for restarting the supply of fuel to the engine, the fuel-cut is interrupted to carry out a fuel recovery or restart the supply of fuel to the engine, to thereby prevent engine stalling.
It has been recognized that the vehicle traction control system relying upon fuel-cut tends to cause a problem upon shifting of an automatic transmission, as explained below. For the sake of convenience, it is assumed that the automatic transmission carries out shifting based on a shift map as shown in FIG. 1, for example. Upon occurrence of an acceleration slip of the wheels, the vehicle speed VSP generally measured from the rotational speed of a moving part between the output shaft of the automatic transmission and the drive axle of each wheel increases, for example, from a point A in the second range to a point B in the third range, and a 2 to 3 upshift is carried out in view of such judgment that the automatic transmission has shifted from the second range into the third range.
When such upshift is carried out by the automatic transmission, the engine speed is also lowered by a value corresponding to the resultant lowering of the gear ratio. In particular, when the wheel slip value is so large as to require an increase in number of the cylinders to which fuel-cut is to be effected, the engine speed may be lowered to the fuel recovery speed due to a significant lowering of the engine output power. In this case, there may occur a control hunting such that the output power of the engine is increased and returned to a regular value with which no traction control is to be carried out, to restart the supply of fuel (fuel recovery) which, in turn, causes an acceleration slip of the wheels occurs once again, thereby requiring the traction control by the fuel-cut.
It is a primary object of the present invention to eliminate the above-mentioned problem of the prior art, and provide an improved vehicle traction control system which inhibits an automatic transmission from carrying out a speed change caused by erroneous detection of the vehicle speed due to an acceleration slip of wheels, so as to prevent the engine speed from lowering down to a fuel recovery speed, and thereby prevent occurrence of hunting of the traction control.
To this end, according to the present invention, there is provided a vehicle traction control system for a vehicle wherein output power of an engine is transmitted to wheels through an automatic transmission, said traction control system being adapted to eliminate an acceleration slip of the wheels by a fuel-cut with which fuel supply to the engine is stopped upon occurrence of the acceleration slip, wherein said automatic transmission is inhibited from being shifted upon occurrence of an acceleration slip with which the fuel cut is carried out, so as to hold a current shift range.
The traction control system according to the present invention can prevent an acceleration slip by fuel-cut of the engine upon occurrence of an acceleration sip of the wheels. Moreover, upon occurrence of an acceleration slip for which the fuel-cut should be carried out, the shifting of the automatic transmission is inhibited so as to hold the current shift range as it is. It is thus possible to inhibit an upshift of the automatic transmission due to erroneous detection of the vehicle speed as a result of the acceleration slip of the wheels even if, otherwise, an upshift would be performed as a result of the erroneous vehicle speed. Also, since an upshift of the automatic transmission due to erroneous vehicle speed is inhibited, it is possible to prevent the engine speed from being lowered to a fuel recovery speed as a result of the upshift.
As mentioned above, known traction control system relying upon the fuel-cut involves a problem when the number of fuel-cut cylinders is increased for lowering the output power of the engine for eliminating a large acceleration slip value of the wheels, that hunting of the traction control occurs such that the engine increases its output power which is therefore returned to a regular value that would be obtained in the absence of traction control, since the engine speed is decreased down to the fuel recovery speed by the upshift of the automatic transmission, and the acceleration slip of wheels occurs once again so as to carry out the traction control by the fuel-cut. Such a hunting problem can be effectively eliminated by the traction control system according to the present invention.
It is preferred that the inhibition of the automatic transmission from shifting is carried out if the acceleration slip has a value that is larger than a set value for causing an upshift so as to lower the engine speed to a fuel recovery speed with which the supply of the fuel into the engine is restarted. In this instance, the shifting of the automatic transmission is inhibited only upon occurrence of a large acceleration slip of the wheels causing the upshift with which the engine speed is lowered to the fuel recover speed, but otherwise no shift range holding control is carried out, thereby making it possible to avoid unnecessary shift inhibition control.
The holding of the current shift range may be released if the acceleration slip value is sufficiently lowered, and if driver""s foot is released from an acceleration pedal. In this instance, it is possible to eliminate an abrupt torque increase due to a downshift caused by depression of the accelerator pedal in association with completion of the traction control following the elimination of the acceleration slip.
Alternatively, or additionally, the holding of the current shift range may be released if the acceleration slip value is sufficiently lowered, and if the automatic transmission carries out either an upshift or no shifting during the releasing of the shift range holding. In this instance, it is possible to eliminate an abrupt torque increase due to a downshift carried out by the automatic transmission in association with completion of the traction control following the elimination of the acceleration slip.